1. Field of the Invention
The present invention relates to fuel assemblies for nuclear reactors and more particularly to such assemblies which may be easily disassembled to permit removal and insertion of fuel or poison rods within the assembly.
2. Description of the Prior Art
It is well known that the fuel or fissionable material for heterogeneous nuclear reactors is conventionally contained in a number of thin walled tubes forming elongated fuel rods which may be grouped and joined together into fuel element assemblies. Each reactor has a number of such fuel assemblies therein comprising the reactor core.
It is also known that it is desirable from the standpoint of cost and reactor efficiency, to have fuel assemblies in which defective fuel rods may be replaced or special fuel or burnable poison rods added after the assembly has been irradiated. Such versatility may preclude an entire assembly from being prematurely scraped because of a relatively few defective fuel rods.
Various prior art fuel assemblies have been designed to have such a capacity. Typical of such designs is the one disclosed in U.S. Pat. No. 3,255,091, entitled "Fuel Arrangement for a Nuclear Reactor" of E. Frisch. The Frisch patent discloses a fuel assembly generally referred to as the "canned" type wherein structural support is provided by a peripherally extending elongated frame or can. The assembly includes removable retaining plates, which, when removed, permit access to and removal of individual fuel rods. End plates arranged at opposite ends of the frame remain permanently attached and contain openings, aligned with the fuel rods, through which the rods may be withdrawn.
Another prior art patent which discloses a fuel assembly design which may be disassembled to provide access to the fuel rods in U.S. Pat. No. 3431170 "Nuclear Reactor Fuel Bundle" of J. L. Lass et al. The Lass patent also employs an outer can structure and has a removable upper tie plate, which, when removed, facilitates easy withdrawal or insertion of fuel rods.
Recently, nuclear reactor fuel assemblies have been made wherein structural support is provided by vertically extending hollow tubes which serve as guides for control rod elements and which are secured to upper and lower end plates. The fuel rods are aligned and supported within the structural framework formed by the guide tubes and end plates by spacer grids which are welded to the guide tubes and provide both lateral and, to some extent, axial restraint to the fuel rods. Generally, the end plates and the guide tubes have been made from the same material such as, for example, stainless steel and the connections between these elements have been made by welding the parts together.
More recently, however, some components of such fuel assemblies have been fabricated from the zirconium alloy Zircaloy. The advantages of Zircaloy for this application include its low neutron absorption cross section and its high corrosion resistance to water at the customary operating temperatures of pressurized water reactors. Disadvantages of Zircaloy include its high cost, primarily due to the extreme difficulty in working the material, and the fact that welding of this material must be performed in an inert atmosphere. Also Zircaloy components have relatively poor wear resistance when subjected to relative motions or rubbing against an adjacent component.
As a result it has been found that from both the standpoint of neutron economy and dollar economy, a fuel element assembly fabricated using Zircaloy for the elements within the core active region, and a material such as stainless steel or the nickel, chromium, iron alloy having the trade name Inconel for the remainder of the elements, is a most attractive design. Those elements considered to be in the active region include the vertically extending control rod guide tubes, the fuel rod spacer grids and the cladding of the fuel rods themselves. Elements outside the active region include the upper and lower end plates and the means associated with the end plates for supporting and maintaining the alignment of the fuel assembly within the reactor core.
Because of the dissimilarity of the metals in such a design, welding cannot be used to attach the guide tubes to the end plates and a suitable mechanical joint is required. An arrangement for forming such a connection is shown in FIG. 4 of copending application Ser. No. 145,374, Fuel Assembly Holddown Device of R. H. Klumb et al. filed on May 20, 1971 and assigned to the same assignee as the present application. While such an arrangement provides a suitable connection, it is not possible to easily disassemble this connection in order to gain access to the fuel rods. Such disassembly becomes virtually impossible, following irradiation of the fuel assembly and the axial thermal growth of the fuel rods which occurs as a result thereof. As a result of the thermal growth, there is very little clearance between the upper ends of the rods and the bottom of the upper end plate and access to and disassembly of the joint is not possible.